This application is based on and claims priority to Japanese Patent Application No. 2000-27561, filed Feb. 4, 2000, the entire contents of which is hereby expressly incorporated by reference.
1. Field of the Invention
This invention relates to a valve cam mechanism for a four-cycle engine, and more particularly to an improved valve cam mechanism that decompresses a combustion chamber for easy starting of a four-cycle engine.
2. Description of Related Art
All internal combustion engines have starting mechanisms. In many applications, the starting mechanism is an electrical device that is operable with a key by the operator. This device provides an easy way to start the engine. Some simpler engines, however, use a manual starting device in order to keep the engine compact. For instance, it is frequently the practice in outboard motors, and particularly those of small displacement, to incorporate a mechanism whereby the engine may be manually started. This is normally done by a rope or recoil starter that is associated with a flywheel disposed atop the crankshaft of the engine.
In order to achieve good engine performance, it is a practice to use relatively high compression ratios. The use of such high compression ratios, however, gives rise to a rather large force that must be overcome by the operator to effect manual starting. There have been, therefore, proposed types of decompression mechanisms which effectively lower the compression ratio of the engine during manual starting. Preferably, such devices are automatic in nature wherein the compression ratio is lowered only long enough to facilitate starting and not long enough to interfere with the operation of the engine once starting has been accomplished. That is, the decompression mechanism should be released promptly when engine is started and the decompression mechanism should not operate above a selected idle engine speed.
Occasionally, engines such as those provided on, for example, outboard motors and lawn mowers have camshafts extending generally vertically. A proposed decompression mechanism for these engines has a construction in which an actuator is mounted on a camshaft for pivotal movement about a pivot axis extending generally normal to an axis of the camshaft. The actuator may have a holder section which is arranged to hold, for example, an exhaust valve in an open position when the actuator exists in an initial position. If the engine has a rocker arm which is periodically lifted by the camshaft to actuate the exhaust valve, the holder section can hold the rocker arm instead of directly holding the exhaust valve. The actuator also has a weight section that places the holder section in the initial position by the gravity, i.e., by its own weight, and moves by centrifugal forces, which are produced by camshaft rotation, to release the exhaust valve such that the exhaust valve can properly close.
When the holder section is in the initial position, the exhaust valve is held in the open position in which a combustion chamber of the engine communicates with the atmosphere. Because no compression force or a reduced compression force is developed in the combustion chamber under this condition, the operator can manually start the engine. Once the engine starts, the camshaft rotates and the actuator is released from the initial position by the movement of the weight section. The combustion chamber no longer communicates with the atmosphere under this condition and normal running of the engine is thus assured. U.S. Pat. Nos. 4,453,507 and 5,150,674 disclose decompression mechanisms in which actuators directly hold valves during engine starting.
Typically, the camshaft defines an aperture extending generally normal to the axis of the camshaft. A shaft of a decompression mechanism is provided through the aperture so as to pivotally mount the actuator on the camshaft. The shaft is press-fit into the aperture. This press-fitting, however, may cause the camshaft to be bent slightly when the shaft is inserted. Thus, the camshaft is no longer straight.
One reason that the camshaft is susceptible to bending is that the shaft must be cold press fit rather that hot press fit. As will be appreciated, the camshaft has cam lobes that comprise a heat treated layer to protect the cam lobes from abrasion during operation. Because hot press fitting can harm the heat treated layers, cold press fitting must be used. Cold press fitting, however, requires a relatively large load as compared to than hot press fitting. The large load can cause deformation of the camshaft.
In addition, camshafts comprise a number of portions that have rather tight dimensional tolerances, such as cam lobes and journals. Thus, the camshaft generally cannot be supported in these regions during press fitting. Instead, the camshaft must be supported elsewhere. These supported regions, therefore, are greatly spaced from the region in which the actuator is being mounted. Accordingly, the bending moment exerted upon the camshaft is greatly increased.
Therefore, a need exists for an improved valve cam mechanism that can substantially reduce any undesired camshaft deformation during installation of a decompression mechanism.
It should be noted that a decompression mechanism that is mounted on a camshaft can suffer from another problem. Namely, chips produced during machining processes or wear dust accumulated during engine operation can adhere onto the coupling portions of the actuator with the decompression shaft. If this occurs, the shaft may not pivot smoothly and the desired operation of the decompression mechanism may fail.
Another need, thus, exists for an improved valve cam mechanism that can maintain smooth operation of a decompression mechanism without requiring special maintenance operations or frequently cleaning.
In accordance with one aspect of the present invention, an internal combustion engine comprises a cylinder block defining at least one cylinder bore. A piston reciprocates in the cylinder bore. A cylinder head member closes one end of the cylinder bore and defines a combustion chamber with the cylinder bore and the piston. An intake passage has an intake port through which air is introduced into the combustion chamber. An intake valve is arranged to open and close the intake port. An exhaust passage has an exhaust port through which exhaust products are discharged from the combustion chamber. An exhaust valve is arranged to open and close the exhaust port. At least one camshaft is arranged for rotation and has cam lobes to activate at least one of the intake valve and the exhaust valve. The camshaft comprises an aperture that extends therethrough and that is positioned next to at least one of the cam lobes. The aperture extends generally normal to an axis of the camshaft. A decompression mechanism is configured to decompress the combustion chamber for manual starting of the engine. The decompression mechanism includes a shaft extending through the aperture. An actuator is affixed to the shaft for pivotal movement about an axis of the shaft. The actuator has a first section arranged to hold at least one of the intake valve and the exhaust valve in an open position when the actuator exists in an initial position. A second section is arranged to initially retain the actuator in the initial position and to release the actuator from the initial position when the camshaft rotates. The shaft is rigidly supported by an inner surface of the aperture. A space is defined between the shaft and at least a portion of the inner surface of the aperture.
In accordance with another aspect of the present invention, an internal combustion engine comprises a combustion chamber. A valve is arranged to open and close the combustion chamber to the atmosphere. A camshaft is arranged for rotation and has a cam lobe to activate the valve. The camshaft defines an aperture adjacent to the cam lobe. The aperture extends generally normal to an axis of the camshaft. A pin extends through the aperture. An actuator is affixed to the pin for pivotal movement about an axis of the pin. The actuator includes a first section arranged to hold the valve in an open position when the actuator exists in an initial position. A second section is arranged to initially retain the actuator in the initial position and to release the actuator from the initial position when the camshaft rotates. The pin is rigidly supported by an inner surface of the aperture in part. A space is defined between the pin and the inner surface of the aperture in the rest part.
In accordance with a further aspect of the present invention, a valve cam mechanism for four-cycle engine having a combustion chamber comprises a valve arranged to open and close the combustion chamber to the atmosphere. A camshaft extends generally vertically. The camshaft is arranged for rotation and has a cam lobe to activate the valve. The camshaft defines an aperture adjacent to the cam lobe. The aperture extends generally normal to an axis of the camshaft. A pin extends through the aperture. An actuator is affixed to the pin for pivotal movement about an axis of the pin. The actuator includes a holder section arranged to hold the valve in an open position when the actuator exists in an initial position. A weight section is disposed opposite to the holder section relative to the axis of the pin so as to place the holder section in the initial position. The actuator pivots about the axis of the pin when the weight moves by centrifugal force produced by the rotation of the camshaft so as to release the holder section from holding the valve in the open position. The pin is rigidly supported by an inner surface of the aperture in part. A space is defined between the pin and the inner surface of the aperture in the rest part.
Further aspects, features and advantages of this invention will become apparent from the detailed description of the preferred embodiments which follow.